Information terminals that are commonly used, such as smartphones and tablets, are capable of displaying, on a display unit, document images in word, text, and PDF formats in which printed documents such as newspaper, magazines, advertisements, and catalogues are digitized. Information terminals have function to change display state of document images (e.g., a document image in PDF format) displayed on display units in response to a user's operation. For example, when a user slides his/her finger while touching a display screen of an information terminal, a document image is scrolled, and when the user touches the display screen with two fingers and widens or narrows the interval of the fingers, the document image is rescaled.
Patent Literature 1 describes a technology for digitizing information provided in print media, such as magazines, advertisements, and catalogues, and displaying the information on a display unit of an information terminal.